This invention relates to a magnetoresistance effect element, a magnetic head, a magnetic reproducing apparatus, and a magnetic memory and more particularly, to a magnetoresistance effect element which has a structure where a sense current is passed perpendicularly to a film plane of the magnetoresistance effect film, and to a magnetic head using the same, a magnetic reproducing apparatus and a magnetic memory.
In a certain kind of ferromagnetic substance, a phenomenon in which resistance changes according to strength of an external magnetic field is known, and this is called the “magnetoresistance effect.” This effect can be used for detection of an external magnetic field, and such a magnetic field detecting element is called “the magnetoresistance effect element (it is also hereafter called “MR element”).”
Such a MR element is industrially used in a magnetic recording and reproducing apparatus, and used for read-out of information which is stored in a magnetic recording medium, such as a hard disk and magnetic tape, (for example, IEEE MAG-7,150 (1971)), and such a magnetic head is called a “MR head.”
In recent years, in a magnetic recording and reproducing device with which these MR elements are used, especially in a hard disk drive unit, the magnetic storage density is increased. As a result, the recording bit size on the medium becomes smaller, and the quantity of leaking magnetic flux from the recording bit, i.e., a signal magnetic field, is being smaller.
Therefore, it is becoming indispensable to realize a MR element which has a higher S/N ratio and higher sensitivity by obtaining a resistance rate of change at a lower magnetic field. This development serves as important base technology for improvement in storage density.
Here, “high sensitivity” means that an amount of resistance change (Ω) per unit magnetic field (Oe) is large. A MR element having a larger MR changing rate and being excellent in the soft magnetic characteristic becomes more sensitive. In order to realize a high S/N ratio, it is important to reduce a thermal noise as much as possible. For this reason, it is not desirable that resistance of the element itself becomes large. For example, when using as a reading sensor for a hard disk drive, a resistance of about 5 ohms–30 ohms is desired as the element resistance in order to realize a good S/N ratio.
From the background, it is becoming common to use the spin valve (spin-valve) film which can obtain big MR rate for a MR element used for a hard disk drive.
FIG. 12 is a conceptual diagram which illustrates the sectional structure of a spin valve film. That is, the spin valve film 100 has the structure where a ferromagnetic layer F, a non-magnetic layer S, a ferromagnetic layer P, and an antiferromagnetic layer A are laminated in this order. Two ferromagnetic layers F and P which sandwich the non-magnetic layer S therebetween are magnetically uncoupled.
Magnetization of one ferromagnetic layer P is fixed in one direction by an exchange bias which is applied from the antiferromagnetic layer A. A magnetization of the ferromagnetic layer F is made rotatable easily by external magnetic fields (signal magnetic field etc.). By an external magnetic field, only magnetization of the ferromagnetic layer F can be rotated, and thus, the relative angle of the magnetization direction of two ferromagnetic layers P and F can be changed. As a result, a large magnetic resistance effect can be acquired (Phys. Rev. B, Vol. 45, 806 (1992), J. Appl. Phys. Vol. 69, and 4774 (1991)).
Here, the ferromagnetic layer F is called a “free layer”, a “magnetic field reception layer”, or a “magnetization free layer” in many cases. The ferromagnetic layer P is called a “pinned layer” or a “magnetization fixed layer.” The non-magnetic layer S is called a “spacer layer”, an “interface adjusting intermediate layer”, or a “intermediate layer.”
In the case of a spin valve film, magnetization of the free layer F, i.e., a ferromagnetic layer, rotates easily even in a lower field. Therefore, a raise in sensitivity is possible and it is suitable for MR element for MR heads.
To such a spin valve element, in order to detect change of resistance by a magnetic field, it is necessary to pass a “sense current.”
Conventionally, a sense current is generally passed in parallel to the film plane, and resistance of a parallel to the film plane is measured. This method is generally called the “CIP (current-in-plane)” system.
In the case of a CIP system, it is possible to acquire about 10–20% of value as a MR rate of change. In the magnetoresistance effect head of the shield type currently used, since a spin valve element is used in the plane form almost near a square, resistance of MR element becomes almost equal to the plane resistance value of MR film.
Thus, in the case of the spin valve film of a CIP system, it becomes possible by setting a field resistance value to 5 ohms–30 ohms to acquire the good S/N property.
Resistance of this level can be realized comparatively easily by making thickness of the whole spin valve film thin. For this reason, at present, the spin valve film of a CIP system is generally used as a MR element for MR heads.
However, in order to realize information reproduction with high storage density which exceeds 100 Gbits/inch2, it is expected that the value which exceeds 30% as a MR rate of change is needed. However, it is difficult to acquire the value which exceeds 20% as a MR rate of change by the conventional spin valve film. Therefore, it has been a big technical subject for further improvement in the storage density how this MR rate of change can be increased.
From such a viewpoint, the spin valve which inserted the “electronic reflective layer” into the pinned layer or the free layer in the CIP-SV film is proposed in order to increase MR rate of change. As the electronic reflective layer, an oxide, a nitride, a fluoride, or a boride can be used.
For example, an electronic reflective layer can be inserted into the pinned layer and the free layer, respectively. By a spin valve film, if electronic scattering takes place at the interface of each layer, a mean free path will decrease, and MR rate of change will decrease. On the other hand, by providing the electronic reflective layer ER to reflect electrons, the mean free path of electrons is made to increase, and it becomes possible to obtain large MR rate of change.
Moreover, in the case of this structure, the probability that an electron will pass through the interface of a magnetic layer/nonmagnetic layer also goes up by reflecting electrons. For this reason, it becomes possible to acquire the same effect as the case in an artificial lattice film, and MR rate of change increases.
However, also in this structure, since all electrons may not pass through the interface of a magnetic layer/nonmagnetic layer, there is a limit in increase of MR rate of change. For this reason, it is substantially difficult to realize large MR rate of change which exceeds 20% and a practical amount of resistance change of 5 ohms–30 ohms in the CIP-SV film which has the electronic reflective layers.
In contrast to this, a magnetoresistance effect element of a structure of passing sense current perpendicularly (current perpendicular to plane: CPP) to a film plane in the artificial lattice where magnetic layers and non-magnetic layers are laminated is proposed as a method of obtaining large MR which exceeds 30%. (hereafter called a “CPP type artificial-lattice”).
With a CPP type artificial lattice type magnetoresistance effect element, electrodes are provided in the upper and lower sides of the artificial lattice where the ferromagnetic layers and the non-magnetic layers are laminated by turns, respectively, and sense current flows perpendicularly to the film plane. With this structure, the probability that sense current will cross a magnetic layer/non-magnetic layer interface becomes high. Therefore, it becomes possible to acquire the good interface effect, and big MR rate of change is obtained.
However, in such a CPP artificial lattice type film, it is necessary to measure a resistance perpendicular to the film plane of the artificial lattice SL which consists of a laminated structure of very thin metal films. This resistance will generally turn into a very small value. Therefore, with the CPP type artificial lattice, it has been an important technical subject to enlarge resistance as much as possible. Conventionally, it was indispensable to make the connected area of the artificial lattice SL and Electrodes EL as small as possible, and to increase the number of laminations of the artificial lattice SL, and to increase the total thickness, in order to enlarge the resistance.
For example, when pattering of the form of an element is carried out at 0.1 micrometer×0.1 micrometer, if Co layers (thickness:2 nm) and Cu layers (thickness:2 nm) are laminated 10 times by turns, the total thickness becomes 20 nm and the resistance of about 1 ohm may be obtained. However, the resistance is not large enough.
If it is considered from a viewpoint of resistance, it is indispensable to make it the artificial lattice type instead of a spin valve type, in order to obtain sufficient head output and to use as a good reading sensor for hard disks in a CPP type structure.
On the other hand, to use MR element for a MR head, it is necessary to control the magnetization of a magnetic layer and measure an external magnetic field efficiently. At the same time, it is required to form each magnetic layer into a single magnetic domain so that a Barkhausen noise etc. may not occur. However, as mentioned above, it is needed to laminate a magnetic layer and a non-magnetism layer repeatedly by turns in order to earn resistance, and it is technically very difficult to control magnetization by CPP type MR element individually to such many magnetic layers.
Moreover, when using MR element for a MR head, it is necessary to make the magnetization rotate very sensitively to a small signal magnetic field to that a large MR rate of change is obtained. For that, it is necessary to increase the signal magnetic-flux density in a sensing portion, so that a larger amount of magnetization rotations are obtained by the same magnetic-flux density. Therefore, it is necessary to make the total Mst (magnetization×film thickness) of the layers where magnetization rotates by an external magnetic field small. However, with a CPP type MR element, in order to earn resistance, it is necessary to laminate magnetic layers and non-magnetic layers repeatedly by turns. For this reason, Mst increases and it becomes difficult to raise the sensitivity to a signal magnetic flux.
Thus, in spite that MR rate of change in a case of a CPP artificial lattice type film exceeds 30%, the sensitivity needed as a MR sensor for magnetic heads is not obtained.
On the other hand, adopting a CPP system in the spin valve structure using FeMn/NiFe/Cu/NiFe, FeMn/CoFe/Cu/CoFe, etc. is also considered.
That is, a sense current is perpendicularly passed to a film plane to the laminated structure which has spin valve structure. However, in such CPP type SV structure, in order to enlarge resistance, it is necessary to thicken thickness of a magnetic layer to about 20 nm. Even in such a case, a resistance rate of change is only about 30% in 4.2K, and is predicted that only about 15% of resistance rate of change of the half may be obtained in room temperature.
That is, by the spin valve film of a CPP system, only about 15% of MR rate of change is obtained. And Mst of a free layer must be enlarged. Therefore, sensitivity required as a MR sensor for heads may not be obtained.
As explained above, various structures, such as a spin valve of a CIP type spin valve film, a CPP type artificial lattice, and a CPP type, are proposed. However, the present magnetic storage density is continuing the rise of an annual rate of 60% or more, and the further output increase will be needed from now on. However, the spin valve film which can be used with high storage density which exceeds 100 Gbits/inch2 at present and which has suitable resistance and the large amount of MR change, and serves as high sensitivity magnetically is difficult to realize.